Gene expression and the control of malignancy will be studied in established mammalian cells grown in vitro. This will be pursued in three stages: (1) the continuation of ongoing efforts including characterization of auxotrophic, drug-resistant and differentiated cell lines; (2) the development of a new experimental system, long-term cultured mammary gland cells; (3) the use of these characters in a program designed to produce an engineered cell line, starting with an established cell line with a very low chromosome number and fusing into it a variety of traits from the several established lines mentioned above. An artificially developed cell line of this sort should be extremely useful for obtaining critical evidence bearing on the nature of differentiation and of the genetic regulation of differentiation and malignancy.